1. Field of the Invention
The present invention relates to a display, and more particularly, to an organic electroluminescent (EL) device and a method for fabricating the same.
2. Discussion of the Related Art
Organic EL devices, also called organic light emitting diodes (LEDs), are becoming very popular because of their possible application to flat panel displays (FPDs). They are extremely thin, matrix-addressable and can be operated at a relatively low voltage, typically less than 15 volts. Furthermore, they have additional characteristics suitable for next generation FPDs such as little dependence on viewing angle and good device-formability on flexible substrates among other things. Organic LEDs differ fundamentally from conventional inorganic LEDs. While the charge transfer in inorganics is band-like in nature and the electron-hole recombination results in the interband emission of light, organic films are generally characterized by the low-mobility activated hopping transport and the emission is excitonic. Organic EL devices are also substantially different from conventional inorganic EL devices, especially in that organic EL devices can be operated at low DC voltages.
A substantial amount of research has been directed toward the efficiency improvement and color control of organic LEDs. The efficiency of some organic EL devices has now been demonstrated to be close to its theoretical limit and certainly adequate for many commercial applications. Moreover, the color control is probably not limiting for many potential applications.
In light of this, we believe that the outlook for commercial applications is excellent for organic EL devices. Their performance is quite satisfactory for many applications. It is valuable to think in terms of specific products and manufacturing techniques for the commercialization of organic EL devices. Consideration of the specific applications leads us to believe that more work on manufacturability, uniformity, reliability, and systems issues is required to commercialize organic EL devices.
Pixellation or patterning, especially of electroluminescent and second electrode materials, is one of the key issues to be solved before the commercialization of organic EL devices. The use of many conventional pixellation techniques is precluded due to the nature of organic materials which are extremely vulnerable to the attack by most solvents. Various efforts have been made to utilize shadowing effects for the film patterning. One approach was introduced by C. Tang et al. in U.S. Pat. No. 5,294,870, wherein partition walls were used as a means to give a shadowing effect. Therein each stripe of the second electrode is supposed to be separated by depositing the second electrode material onto the transparent support with relatively high partition walls at an oblique angle with the surface normal. The idea may be conceivable, but is not suitable at all for mass production. First of all, it is impossible to have a vapor source which may set the direction of traveling metallic vapor at a constant angle with the surface normal over the whole panel surface. Whether one has either a point source or a large area source, the angle would vary with the position of pixel on the transparent support because of the diverging nature of vapor. One may be able to obtain a certain degree of separation with a complicate arrangement of source and support, but only at the sacrifice of production yield. More practical processes have been proposed by K. Nagayama et al. in U.S. Pat. No. 5,701,055 wherein electrically insulating ramparts, like a bunch of tiny umbrellas, serve as a means to shadow the incoming vapor. An advantage of U.S. Pat. No. 5,701,055 over U.S. Pat. No. 5,294,870 is that depositions do not need to be made at an oblique angle, which markedly widens the process window.
FIGS. 1a.about.1d illustrate typical process steps proposed in U.S. Pat. No. 5,701,055. Referring to FIG. 1a, an array of anode stripes 2 is formed of typically indium tin oxide on a transparent substrate 1, and insulating patterns 3 are formed thereon. As shown in FIG. 1b, next, each of ramparts 4 is formed to fit into the small gap between a pair of the insulating patterns 3. The rampart 4 with a pair of overhangs 8 serves an important role to shadow the incoming vapor. When organic materials and a second electrode material are vapor deposited, the films are formed on top of the ramparts as well as on the surface between the ramparts, as seen in FIG. 1c. The important point is that the films are not formed on the side surface of the ramparts due to a shadowing effect, which ensures the electrical insulation between any two adjacent pixels. After the formation of an electroluminescent multilayer 5 and a second electrode 6 on the first electrode 2 in succession, an encapsulation layer 7 is formed, as in FIG. 1d. In summary, the background art of U.S. Pat. No. 5,701,055 appears to be an effective way to pixellate an organic electroluminescent display panel.
However, the background art EL device has several drawbacks as follows.
(1) While a variety of ramparts with the different shape is shown in U.S. Pat. No. 5,701,055, the most typical one looks like that shown in FIG. 1b. Since this type of rampart 4 easily causes short-circuits between the first and second electrodes at the edges of the second electrode stripe, it is often necessary to form additional insulating patterns 3. But it is not necessary to fabricate them in the present invention.
(2) Ramparts are often formed of photoresist and the most important parts of the m are the overhangs 8 which actually shadow the incoming vapor. It is quite a delicate process to make ramparts with good overhangs. The present invention provides ramparts, or partition walls, the production of which can be much simplified.
(3) Ramparts remain standing on the panel surface even after the completion of the device fabrication. A part of rampart itself or films on top of them may fall down to damage the device locally during the subsequent fabrication processes. To avoid this kind of potential problems, it is recommended to have a stable structure: the shape of trapezoid in the present invention is structurally more stable compared to the inverted trapezoid of the related art.